1. Technical Field
The present invention relates to processing performed in a GPS receiver and, more particularly, to coherent processing performed prior to correlation processing.
2. Related Art
In a GPS receiver, the following correlation processing is performed. That is, for C/A (Clear and Acquisition or Coarse and Access) codes constituting a received GPS signal, C/A codes generated on the basis of an internal timer of the GPS receiver are sequentially integrated while shifting time. A shift amount at time when an integration result is maximum is defined as arrival time of the GPS signal. However, when the intensity of the GPS signal is weak (i.e., when the intensities of the C/A codes are weak), the maximum integration result cannot be specified in the integration results. For this reason, in a high-sensitive GPS receiver, in order to specify the maximum integration result in the integration results even when the integration of a GPS signal is weak, coherent processing which accumulatively adds C/A codes to increase the intensities of the C/A codes is performed prior to correlation processing.
However, for C/A codes serving as targets of the coherent processing, a navigation message such as almanac data or ephemeris data is coded. In the navigation message, C/A codes of 20 cycles are set as 1 bit. Therefore, the C/A codes maybe inverted in polarity every 20 cycles. When the C/A codes are simply accumulatively added, cancellation is caused by the polarity inversion, and high sensitivity cannot be achieved.
Therefore, as a related art, the following inventions are proposed (JP A-2000-193735 and JP A-2001-349935). That is, before C/A codes are accumulatively added to each other, polarity inversion of the C/A codes is corrected on the basis of a navigation message modulated in a received GPS signal or an externally acquired navigation message.
However, it takes a long time to acquire a navigation message from a GPS signal.
And for example, as in the third-generation mobile communication scheme (3G) or the like, an external navigation message may not be provided to a GPS receiver.